Fine ultra structure of the tip-linkage: The importance of the filamentous tip-linkage in the transduction process of the hair cell is well-recognized. The destruction of the tip-links by a calcium chelator, BAPTA, abolished transduction current. We have examined tip-links in the sensory hair cells of the guinea pig organ of Corti using freeze-fracture and deep-etching techniques. The data suggest that the filamentous tip- links are formed by two or three twisted fine filaments emanating from the tip of the shorter stereocilia and attached to the side surface of the neighboring taller stereocilia. The tip-links at this attachment are often bifurcated, suggesting that more than one transduction channel per stereocilium at the insertion sites. Fine structure of the tectorial membrane: It is known that stereociliary bending produced by the shear motion between the tectorial membrane and underlying stereocilia is an important first step for auditory sensory transduction. We examined the tectorial membrane using high resolution transmission and scanning electron microscopy, rapid-freezing, freeze- fracture, deep-etching and replica techniques. The type A fibrils of tectorial membrane are formed of bundled collagen fibrils interconnected by smaller filaments. Their orientation coincides with the angulation of the stereociliary formation. Considering the predicted stiffness of the tectorial membrane and stereociliary bundles, this structural organization is compatible with the concept that the tectorial membrane motion would modulate the subtentorial space fluid drag and in turn modulate inner hair cell stereociliary motion. High resolution scanning electron microscopy with osmium digestion techniques was used to study the three dimensional cytoplasmic organization of the hair cells. The complex membrane organization of the endoplasmic reticular system indicates functional divisions among different compartments of the membrane system. Numerous vesicles are attached to the cytoskeletal filaments near the cytoplasmic membrane suggesting that the microfilaments may play a role in the vesicular trafficking.